Ecological consequences of early Late Pleistocene megadroughts in tropical Africa

Abstract

Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416-16421]. This resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an approximately 125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (approximately 35-15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa.

Fig. 1.

Screen-wash records vs. time and depth for core 1C and PCA first axis of variance (strongly correlated with lake level) vs. time and depth. mblf, meters below lake floor. The probable paleosol at 108.5 ka indicates a minimum lake-level decline: The lake may have dropped below the core site elevation.

Fig. 2.

Stratigraphic profile of select diatom species and taxonomic groups and DCA1. Horizontal lines indicate diatom zones from constrained cluster analysis. A dendrogram is plotted to the right of the diagram.

Fig. 3.

Summary palynostratigraphy of core 1C. (See SI Table 4 for details and additional taxa.) Other montane taxa include Olea, Kiggelaria, Ilex, Juniperus, Apodytes, and Syzygium. Brachystegia and Uapaca plots are shown with actual percentage values and at 5× exaggeration.

Fig. 4.

Comparison of major summary paleoecological records for core 1C. From left to right: PC1 for screen-wash data, DCA1 for diatom data, total PARs, and rainy season solar insolation (W/m²) for the drill site.